I understand the desire to shorten the overly-descriptive title down to a few words, but the current translated title "Slave of Black Knight" is not even slightly faithful to the original title.
*linguistic nerdiness warning*
First of all, the currently translated title implies that someone (protagonist?) is the slave of a knight wearing black armor, or a "black knight". That's not even close to what the original title says.
The original title says that the protagonist is figuratively a slave to a "black" order of knights (I say figuratively because it's pretty clear by the rest of the title that he's at no obligation to stay there). They use the English word "black" here, which really means "engaged in unethical or illegal practices". In other words the protagonist is being treated like a slave by an organization of knights that doesn't care about his well-being; not a knight in black armor. That's the first part of the title that was translated in an unfortunate way.
Of course it follows the trend of overly-descriptive titles though, so it continues on that the protagonist is taken in by a more ethical guild and immediately promoted to "S rank".
Anyway what I'm trying to say is, maybe reconsider how you translate the title.